Bridges should not perform DNS self-tests

From run_scheduled_events in src/or/main.c:

  /** 9. and if we're a server, check whether our DNS is telling stories to
   * us. */
  if (is_server && time_to_check_for_correct_dns < now) {
    if (!time_to_check_for_correct_dns) {
      time_to_check_for_correct_dns = now + 60 + crypto_rand_int(120);
    } else {
      dns_launch_correctness_checks();
      time_to_check_for_correct_dns = now + 12*3600 +
        crypto_rand_int(12*3600);
    }
  }

is_server in the outermost if statement's condition should be public_server_mode(options).